A ferroelectric memory (FRAM (a registered trademark of USA, Ramtron International Corporation)) has drawn attention as a nonvolatile memory capable of holding data even a power source is turned off.
Used as material of a capacitor dielectric film of a ferroelectric capacitor are PZT-based material such as Pb(Zr,Ti)O3 (PZT) and La-doped PZT (PLZT), and Bi layer-structured compound such as SrBi2Ta2O9 (SBT) and SrBi2(Ta,Nb)2O9 (SBTN), which are ferroelectric material. These ferroelectric materials are easy to be reduced by hydrogen. Manufacture processes for a ferroelectric capacitor include a process during which hydrogen is generated, such as a process of forming an interlayer insulating film. In order to prevent a dielectric film of a ferroelectric capacitor from being reduced by hydrogen, it is preferable to dispose a capacitor protective film having a hydrogen barrier function over the ferroelectric capacitor. In a ferroelectric capacitor having a MOS transistor, a gate length of which is equal to or longer than 0.35 μm, an aluminum oxide (hereinafter called “ALO”) film deposited by sputtering has been used generally as a capacitor protective film.
By setting a density of an ALO film covering the ferroelectric capacitor equal to or higher than 3.0 g/cm3, it is possible to prevent the electrical properties of the ferroelectric capacitor from being degraded even if heat treatment in a reductive atmosphere is performed (Japanese Laid-open Patent Publication No. 2001-44375). An ALO film is formed by RF sputtering, ECR sputtering or inductively coupled RF plasma enhanced sputtering, using an aluminum target. While the ALO is formed by sputtering, hydrogen will not be generated so that the ferroelectric capacitor is not degraded while the ALO film is formed.
Annealing is performed after a ferroelectric capacitor is covered with a first-layer ALO film (Japanese Laid-open Patent Publication No. 2002-94021). An interlayer insulating film is formed on the first-layer ALO film, and a local wiring is formed on the interlayer insulating film. The local wiring is connected to an upper electrode of the ferroelectric capacitor via an opening formed through the first-layer ALO film and the interlayer insulating film.
A second-layer ALO film is formed on the local wiring. By forming the two or more than two ALO films, it is possible to prevent the ferroelectric capacitor from being degraded while multi wiring layers are formed. It is possible to prevent the imprint characteristics inherent to a ferroelectric memory from being degraded.
For a ferroelectric memory of the next generation having a MOS transistor, a gate length of which is about 0.18 μm, a stack capacitor structure is adopted to improve an integration degree. The “stack capacitor structure” means the structure that one impurity diffusion region of a MOS transistor is directly connected to a lower electrode of a ferroelectric capacitor via a conductive plug formed through an interlayer insulating film in a thickness direction.
In order to realize extremely high integration, it is desired to etch the layers from an upper electrode to a lower electrode of a ferroelectric capacitor in one etching process. This one etching process results in a large aspect ratio of a lamination structure of the layers from the upper electrode to the lower electrode of the dielectric capacitor. It is apprehended that a conventional ALO film formed by sputtering has insufficient coverage (step coverage) of a step of a ferroelectric capacitor having a large aspect ratio.
Step coverage is able to be improved if an ALO film is formed by chemical vapor deposition (CVD). An ALO film is able to be formed by atomic layer deposition (ALD) which is one type of CVD. In forming an ALO film by ALD, trimethylaluminum (TMA) is generally used as aluminum source material, and water (H2O) is used as oxidant.
First, water is adsorbed to the surface of a substrate, and unnecessary water is purged by evacuating the inside of a chamber. Next, TMA is supplied onto the substrate to make TMA react with OH groups and form Al2O3 of one atomic layer. Redundant TMA is purged by evacuating the inside of the chamber. An ALO film is formed by repeating a set of these cycles.
As an ALO film to be used as a protective film for a ferroelectric capacitor is formed by the above-described method, moisture and hydrogen are adsorbed in the ferroelectric film. Alternatively, hydrogen remained in the ALO film reduces the ferroelectric film in a subsequent process. The ferroelectric capacitor is therefore degraded and does not function as a memory in some cases.
An ALO film may be formed by using TMA and ozone (O3) (Japanese Laid-open Patent Publication No. 2004-193280). Since O3 not containing hydrogen is used as oxidant for TMA, hydrogen is prevented from being adsorbed in the ferroelectric film and remaining in the ALO film. It is therefore possible to prevent the ferroelectric capacitor from being degraded.
An ALO film is formed on an interlayer insulating film having a planarized surface (Japanese Laid-open Patent Publication No. 2006-49795). Since the surface of the layer below the ALO film has no step, even an ALO film formed by sputtering and having low step coverage is able to demonstrate sufficient barrier performance against water and hydrogen. It is possible to alleviate difficulty of etching an ALO film by forming the ALO film divided into several ALO films.